Hot press forming is recently adopted as a steel sheet forming means for an automobile component or the like using a high-tensile steel sheet. In hot press forming, as a result of press forming a steel sheet at a high temperature, forming is carried out in a stage where a deformation resistance is low, and quench hardening by rapid cooling is done, and therefore, it is possible to obtain a component or the like which has a high strength and a high shape accuracy, without generating a forming defect such as a deformation after forming.
In hot press forming, a steel sheet having been heated to a predetermined temperature by a heating furnace in advance is supplied to a mold, and in a state where the steel sheet is placed on a die or floated by a jig such as a lifter built in the mold, a punch is lowered to a bottom dead center, and then a refrigerant such as water, for example, is supplied to between the steel sheet and the mold to cool the steel sheet rapidly. Therefore, a surface of the mold is provided with a plurality of independent projecting portions with a constant height and the inside of the mold is provided with a channel of water communicated with ejection holes of the refrigerant which are provided in a plurality of places in the surface of the mold and a channel for sucking the supplied water. In a conventional cooling method for hot press forming of a thin steel sheet, since the same flow amount is kept while cooling is carried out by flowing cooling water, the same ejection amount is ejected from each ejection hole during a cooling time period.
In a case where hot press forming is carried out by using a mold of such a configuration, it is considered to shorten a cooling time period by increasing a flow amount of cooling water, in order to further improve a productivity. However, it is found that a variation of qualities such as a formed shape (warpage) and a quenching characteristic occurs depending on a region. This is caused by nonuniformity of cooling due to a difference in cooling speed by the flow of the refrigerant in a neighborhood of the ejection hole and its periphery. In other words, the difference in cooling speed generates a thermal stress, which causes the quality to vary. Further, as a result of further study by the inventors, it is found that there is cooling unevenness in a circular state centering on the ejection hole. It is considered that if cooling water is ejected at a predetermined ejection amount from the beginning of cooling, bumping or entrainment of air occurs concentrically centering on the ejection hole, thereby to generate cooling unevenness. Therefore, a device of some kind is necessary with regard to an amount supplied of the refrigerant.
Note that the applicant has already suggested a hot press forming method of Patent Literature 1 with regard to supply control of a refrigerant in a hot press forming method. In the above hot press forming method, a heated thick steel sheet is placed on a rapid cooling mold, the refrigerant is supplied to the thick steel sheet to carry out rapid cooling while the rapid cooling mold is held at a bottom dead center, and thereafter, supply of the refrigerant is controlled in a state where the rapid cooling mold is held at the bottom dead center. More specifically, stopping of supply of the refrigerant and conducting supply of the refrigerant again after a predetermined time period passes is repeated at least once or more, or a predetermined supply flow amount of the refrigerant is once reduced halfway and the supply flow amount of the refrigerant is increased again after a predetermined time period passes.
However, in the hot press forming method of Patent Literature 1, a target steel sheet is what is called a thick sheet and an object thereof is to make a formed product in which a strength is changed in a thickness direction of the steel sheet. Therefore, without a countermeasure, in hot press forming of a thin steel sheet, it is impossible to improve a distortion of a shape of the steel sheet or quality unevenness caused by nonuniformity of cooling due to the aforementioned difference in cooling speed which occurs in a neighborhood of an ejection hole and its periphery.